Major effort of our work this year is centered around the investigation of the electronic structure and the reaction of cytochrome P-450 using a model system as well as mammalian liver microsome. The model system for Fe(III) in P-450, which we reported last year, does not involve any sulfur containing ligand, yet does simulate nicely the characteristic epr (electron paramagnetic resonance) behavior of Fe(III) in native P-450. In order to study the details of the electronic structure of Fe(III), the refinement in the method of analysis of epr data is made this year by retaining all the quartic terms in the spin Hamiltonian. On the reaction side, the benzo(a) pyrene-microsome system is of unique interest in that the reaction yields some free radical. By using 1702 nitrogen mixture in place of ordinary air, we have proved that the reaction is mediated by P-450, the radical is an oxyradical, and the source of oxygen is atmospheric. The free radical signal, however, is mostly due to the non-enzymatic air oxidation of some precursor formed during the incubation. Other systems studied include Co(II) tetraphenylporphine and non-heme iron protein in ENTAMOEBA HISTOLYTICA.